1. Field of the Invention
This invention relates generally to methods and apparatus for the generation of neutrons and gamma rays for interrogation of objects, and more specifically to plasma driven methods and apparatus for the generation of such neutron/gamma rays.
2. Description of the Prior Art
Many non-intrusive active interrogation techniques utilize neutrons or gamma rays to detect special nuclear material (SNM) concealed in cargo. Other applications include oil-well logging, medical imaging, mechanical inspection, etc. For active interrogation systems with neutron sources, neutron induced gamma rays are detected and, sometimes, transmitted neutrons are measured as well. Neutron induced gamma spectra of different materials are used as a type of fingerprint. Fast neutrons are often used to obtain a deep penetration into large inspected objects and, thus, generate a very high background from surrounding materials. While this high background restricts the maximum screening speed of many neutron-based systems, neutrons also tend to activate the surrounding materials after an extensive long period of operation.
Gamma-based systems, on the other hand, detect neutrons produced from photonuclear reactions or transmitted gamma rays. Because the neutron production cross sections of many special nuclear materials due to photofission are much higher than that of most common materials, the neutron background in gamma-based interrogation techniques is fairly low. Furthermore, the induced radioactivity of surrounding materials due to gamma rays of less than 16 MeV is rather small due to the high threshold energy of photonuclear reactions.
The generators used for these sources typically consist of three main components, (1) the ion source, (2) the extraction and acceleration column and (3) the target. The ion source is where the ions are generated. For long life and efficient operation, RF induction discharge is normally employed. Single or multiple ion beamlets are then extracted from the source plasma and accelerated to the desired energy by means of an electrostatic acceleration column. Depending upon the final beam energy and beam shape, the acceleration column can have various configurations and voltage distributions. These ion beamlets impinge on a target which is in the form either of a Ti film for neutron production or a boron B11 compound such as LaB6 for 11.7 MeV gamma production.
Depending on the application, to accelerate the protons generated at the ion source, the number of electrodes in an electrostatic acceleration column can vary from one to five or more. Construction and mechanical alignment of these beam electrodes is not a simple task. Normally, ion optics simulation is required to guide the mechanical design. For voltage hold-off reasons, the length of the column can be relatively long. External pumping is needed to maintain a low pressure inside the column so as to minimize beam loss and secondary electron generation. Electrons formed in the acceleration column and on the target surface will be accelerated back towards the ion source. These backstreaming electrons can cause damage on the electrodes and the ion source chamber. They can also produce substantial amount of x-rays. In addition, they increase the drain current of the high voltage power supply. For this reason various protection schemes are required in the design of the generator to keep these secondary electrons from accelerating back to the ion source chamber.
Most existing gamma-based interrogation systems use electron linacs or microtrons to generate the gamma beams; thus, the deployment of these systems is limited by their size, complexity and high cost of ownership. In commonly owned U.S. Pat. No. 6,870,894, another approach is described covering a compact coaxial system for the generation of neutrons and gamma rays. While such a co-axial apparatus is very useful for the generation of such energy beams, there still remains the need for even lower-cost, more compact, portable gamma and neutron sources for use in active interrogation systems to detect SNM.